1. Field of the Invention
This invention generally relates to the synthesis and recovery of energetic materials, especially for use in gun and rocket propellants and explosives. More specifically, the invention relates to the synthesis and recovery of nitramines.
2. State of the Art
Nitramines are highly energetic compounds having found wide acceptance in the art of explosives and rocket propellant. The most common nitramines in use in the explosives and propellant arts today are 1,3,5-trinitro-1,3,5-triaza-cyclohexane (RDX) and 1,3,5,7-tetranitro-1,3,5,7-tetraaza-cyclooctane (HMX). The acceptance of RDX and HMX in the art is generally attributed to the high energetic performance and the high energy density possessed by these compounds. In essence, RDX and HMX are the standards of energetic performance and energy density by which other energetic compounds are measured.
A drawback to RDX and HMX is that these nitramine compounds are relatively sensitive to shock, friction, and impact. The high sensitivities associated with RDX and HMX make these nitramine compounds less desirable for some applications, especially where the compounds are used or stored in an environment in which traumatic stresses may be encountered. By way of example, in a military conflict, the traumatic impact of hostile ammunition into a rocket motor or weaponry carrying RDX or HMX can lead to destruction of surrounding objects and loss of human life. In the event that the impacted rocket motor or weaponry is in proximity to arsenal or other explosive or combustible materials, catastrophic damage may result.
There is thus a need in the art to provide a highly energetic and high energy density compound that is relatively insensitive to physical stimuli, such as shock, impact, and friction. Various other nitramine compounds have been synthesized towards this end. For example, U.S. Pat. No. 4,085,123 to Flanagan et al. describes the synthesis of the nitramine compound 1,3-diazido-2-nitrazapropane (DANP) as an energetic liquid plasticizer for solid propellants. According to Flanagan et al., the DANP azide plasticizer is synthesized by generating a solution of 1,3-diacetoxy-2-nitrazapropane and dioxane and saturating the solution with anhydrous chloride gas. The use of anhydrous chloride gas makes this process extremely hazardous. In this regard, the Flanagan et al. patent states that the resulting DANP plasticizer is sensitive and must be handled with caution. Moreover, the acetate and chloride precursors are inherently impure, since their reactions reach and remain at equilibrium. In order to minimize the impurities, Flanagan et al. teach distilling both the 1,3-diacetoxy-2-nitrazapropane precursor and its chlorine analogue prior to performing the reactions. On an industrial scale, these distillation techniques are impractical and highly hazardous.
Another nitramine synthesis route is disclosed in U.S. Pat. No. 5,243,075 to Cason-Smith, which describes contacting an N-acetoxymethyl nitramine with a mixture of concentrated hydrochloric acid and trifluoroacetic acid to produce the corresponding N-chloromethyl nitramine analogue. The chlorinated nitramines produced by this process are not sufficiently robust or chemically stable for effective use as an energetic material. The chlorine atoms of the N-chloromethyl nitramines are inherently unstable. When heated, hydrogen chloride gas is released. The presence of the HCl acid can lead to degradation of the material via autocatalysis.
The synthesis of 1,3-bis-(3′5′-dinitro-1′,2′,4′-triazolo)-2-nitrazapentane (BNTN is also known. BNTN has the following structure: 
Specifically, it is known to produce BNTN by suspending sodium dinitro-1,2,4-triazole in dry acetonitrile, and adding 2-nitraza-1,3-dichloropropane. It is believed by the inventors that the 2-nitraza-1,3-dichloropropane is prepared by reacting 2-nitraza-1,3-diacetoxypropane with an inorganic chlorinating agent, such as phosphorus pentachloride (PCl5). According to this method, however, the 2-nitraza-1,3-diacetoxypropane must be purified by distillation prior to chlorination to avoid the formation of chlorine by-products. Another distillation step is needed prior to the reaction of the 2-nitraza-1,3-dichloropropane with the sodium salt of dinitro-1,2,4-triazole. The hazardous nature and toxicity of 2-nitrazapropane and its by-products make distillation highly undesirable and impractical to produce on an industrial scale. Further, the 2-nitraza-1,3,-dichloropropane is very electrophilic and may react with incidental moisture to replace the chlorine atoms and form hydroxymethyl nitramines, thus complicating the nucleophilic addition of the triazole.
The inventors have found that the technique disclosed in the Cason-Smith patent is not suitable for making BNTN. The conditions set forth in the Cason-Smith patent—e.g., treating with HCl and trifluoroacetic acid—were insufficient to drive the reaction of 2-nitraza-1,3-diacetoxypropane and, as a consequence, a largely impure product was obtained.